H. YANG, B. H. ZHANG

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which low frequency and low voltage happen at the same

time.

5)The present UVLS and UFLS may not prevent is-

land from collapse under some situations(e.g., the per-

centage equals 35% or 40% ),in which high frequency

and low voltage happens at the same time, causing that

UFLS cannot act and UVLS is too late to shed insuffi-

cient amount of load.

5. Conclusions

The island frequency response is decided by both me-

chanical power and electromagnetic power. Island volt-

age affects electromagnetic power, which change the

frequency dynamic in island. Initial reactive-power defi-

cit and load characteristics are two key factors affecting

the frequency response dynamic through coupling of

voltage and frequency when initial active-power main-

tains unchanged.

When initial reactive-power or percentage of induction

motor in loads increases, the voltage decreases, leading

the decrease of total electromagnetic power. As the de-

crease of unbalanced active-power in island, the decrease

of frequency decline rate and the increase of frequency.

While initial reactive-power deficit or percentage of in-

duction motor in load exceeds a certain threshold, the

signal of unbalanced active-power in island would

change, which may cause high frequency despite of se-

vere active-power deficit in the island.

Ignoring the coupling of voltage and frequency dy-

namic would result in remarkable errors when analyzing

the frequency response of the splitting island with large

initial active-power deficit.

Moreover, the currently used UVLS and UFLS, whose

setup and performance are independent without consid-

ering the coupling of voltage and frequency, would cause

excessive when low voltage and low frequency happen at

the same time or insufficient control when initial reac-

tive-power deficit or percentage of induction motor in

load exceeds a certain threshold.

It is necessary to develop coordination control for vol-

tage and frequency on the basis of research on their cou-

pling effects.

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